The Chinese cabbage (Brassica rapa L. ssp. Pekinensis) originates in China, which is one of the most important vegetable crops in China and even the world. The leaf head is the main organ of Chinese cabbage for eating, and its size is an important economic property which should be considered in the work of Chinese cabbage breeding at present. It has been demonstrated by many years of production practice that the leaf head size of Chinese cabbage is controlled strictly by genetic and affected by the related gene expression and regulation. It is of important theoretical significance and practical value in controlling leaf head size genetically and improving the yields and quality of crops to start researches on the genes related to the leaf head size of Chinese cabbage.
Although the sizes of plant organs vary considerably between different species, the organs of the individuals within one species possess the relatively uniform size, indicating that the sizes of plant organs are controlled strictly by genetic. Studies have shown that there are multiple genes existing in plant for controlling the sizes of the organs, such as ANT, AtGRF1-AtGRF5, ARGOS, BrARGOS, AtGIF1, STN1, AtMRB1, ANGUSTIFOLIA, AtEXP10, ARL, ROT3, RON2/LUG and BPE genes. They decide the sizes of the organs by regulating cell division and growth. For example, overexpression of AtGRF1 and AtGRF2 in Arabidopsis thaliana led to larger leaves and cotyledons. In contrast, the atgrf1-atgrf2-atgrf3 tri-mutant led to smaller leaves and cotyledons. The phenotypic variations were due to the increase or decrease of the cell volume, indicating that the AtGRF protein regulated the cell extension of the leaves and cotyledons tissues. Because of the decreasing number of cells in the width direction of the leaves, the GIF1 function defect mutant led to narrow leaves and petals, indicating that GIF1 gene controlled the growth and shapes of the leaves and petals.
The AINTEGUMENTA (ANT) function defect mutant in Arabidopsis thaliana reduced the size and number of the leaves and flowers, while the ectopic expression of ANT gene led to larger vegetative organs (such as leaves and stems) and flower organs. ANT gene altered the sizes of mature organs mainly by affecting the total number and the division extent of the cells. The gene did not control cell growth rate and cell cycle, but regulated the organ growth and cell division during organogenesis. These results indicated that ANT gene probably maintained the sustained cell division which was in coordination with growth. The ANT function defect mutant led to reduction of cell mitosis, early termination of cell growth, and accordingly reduction of the organ size. In contrast, the plants with overexpression of ANT gene could make their own cells grow and divide longer than normal cells, and accordingly possess larger organs. The function of ANT gene is not limited to Arabidopsis thaliana, the expression of 35S::ANT also makes the transgene tobacco plants enlarge. ANT gene encodes a transcription factor with an AP2-domain, and its homologues genes have been isolated from other plants. The ectopic expression of rape BANT gene also made the organs of Arabidopsis thaliana enlarge, further indicating that ANT genes from different plants possessed conservative function in controlling the organ size.
ARGOS gene functions upstream of the ANT gene and affects the division ability of the organ cells. The plant organs with positive or negative expression of ARGOS gene would be larger or smaller separately, because the changes of the cell number and organ growth duration led to the alteration of organ sizes. Overexpression of ARGOS-LIKE gene in Arabidopsis thaliana would make plant organs enlarge. The reason was not the increase of the cell number, but the enlargement of the cell size, indicating that the functions of the homologous genes in controlling the organ size were different.
The TCP family of transcription factors is unique in plant. TCP domain gene plays a key role in developmental regulation, and the different members in different species participate in different morphological development processes. In the TCP family, the first member isolated and identified was the Cycloidea (Cyc) gene in snapdragon. Cyc and another TCP family member Dichotoma (Dich) together control the asymmetric development of snapdragon flower. When Cyc and Dich were double mutated, the bilateral symmetrical flower of snapdragon would change to radial symmetrical flower. In corn, the TCP family member Teosinte Branched 1 (Tb1) controls the tillering capacity of corn. The mutation of Tb1 gene led that the apical dominance of the normal wild corn lost and the lateral buds grew and developed uninhibitedly and sequentially developed to be lateral branches, whose phenotype was very similar to teosinte which is the ancestor of corn. Moreover, PCF 1 and PCF 2, the Cyc/Tb1 homologous gene in rice, also have been cloned. Gene sequence alignment shows that there is a highly conserved domain in the sequences of Tb1, Cyc, PCF1 and PGF2, which can form an untypical Helix-Loop-Helix structure and is composed of about 60 residues. This conserved sequence is named TCP domain which is from the first letter of Tb1, Cyc and PCF. Genes who possess the conserves domain are called TCP domain gene. Based on the characteristic of the TCP domain sequence, the TCP gene family is divided by two subgroups: one is represented by PCF, the other is represented by Cyc and Tb1. They show some differences in biological functions. For example, TCP20 gene that belongs to subgroup I can up-regulate the cell growth, while the CIN, TCP2 and TCP4 genes that belong to subgroup II function oppositely and down-regulate the cell growth.
Therefore, if we find and make use of the genes that control the cell growth of Chinese cabbage and substantially control the organ size, it will play a very important role in the improvement of the quality of Chinese cabbage.